lib: rename common sign/verification functions and variables
lib: rename common sign/verification functions and variables

Some gpg-named functions/variables should be used for any signature
system, so remove "gpg_" prefix from them to avoid confusion.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>

Closes: #1889
Approved by: cgwalters

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree